Processes for the polymerization or co-polymerization of ethylene and/or one or more alpha-olefins comprising contacting ethylene and/or the alpha-olefin(s) with a catalyst comprising a first component which is a bis(cyclopentadienyl) Group IVA metal compound containing a substituent capable of reacting with a cation, and a second component which is a compound having a bulky anion containing one or more boron atoms and which is substantially non-coordinating under the reaction conditions and a cation, are well known in the art. Such processes are the subjects of EP-A-277003, EP-A-277004, EP-A-427696 and EP-A-443686. These processes are suitable for preparing polymers having a high molecular weight, as well as for preparing oligomers, which are products with a relatively low molecular weight, for example dimers, trimers and tetramers. In the context of this patent application the term "polymer" is therefore to be understood as including oligomers as well.
Although the catalysts used in these processes are very efficient polymerization catalysts, they possess a disadvantage in that during the course of the polymerization process the catalytic activity may decline to an unacceptably low level, so that the polymerization reaction times or run times are short, for example, up to only a few hours. Thus, the catalysts have a limited life-time and need to be discarded after having been used, because regeneration processes for the catalysts are not available. The disposal of used catalyst may contribute to environmental problems associated with the polymerization process. In addition, the catalysts comprise complex, high-value chemicals so that the catalyst costs may contribute considerably to the total costs of the polymerization process. It is therefore an object of the present invention to provide a process in which a more economic use of at least one of the catalyst components can be made.
It has now been found that when in a polymerization reaction mixture the activity of the catalyst has declined to a low level the activity can substantially be restored by adding a further quantity of the first catalyst component. By this finding, it is possible to make a more efficient use of the second catalyst component, because, for example, remnants thereof can be (re-)used by adding thereto first catalyst component. Thus, the overall supply of the second catalyst component relative to the quantity of polymer to be produced can be reduced to a very low level. It is a further merit of the present finding that the reaction time or run time of the polymerization process can be increased considerably, for example, to 400 hours and beyond.
When the polymerization reaction, and in particular the oligomerization of lower olefins to higher linear alpha-olefins, is performed in a continuous mode, the presence of active catalyst in the product leaving the reactor is detrimental to the product quality since it can further catalyze the isomerization of alpha-olefins to beta-olefins. Therefore, in a continuous-mode polymerization reaction according to the present invention it is recommended to deactivate the catalyst in the product stream leaving the reactor as a first step of product work-up. Such deactivation may be performed by adding water or a lower alcohol such as methanol, in an amount at least equimolar to that of the active catalyst. Subsequent to product work-up, the remaining solid residue can be recirculated to the reactor, together with an additional amount of the first catalyst component, which according to the present invention restores the activity of the catalyst.